Spray-Away - DW Inc.
This is a commercial purpose built replacement for doing exactly what you want to do. I have not used it personally, but it looks like a good solution.
Current TV:'14 F350 Lariat CrewCab, 4x4 Short Bed SRW Diesel w/ Pullrite 2700 (16K)
Current Fiver: '15 Puma 295BHSS Anniversary Edition
Past TV: '13 F150 HD Payload SCrew EB, '08 Tundra CrewMax 5.7L, '02 'Burb 5.3L, '97 Dakota 3.9L
Past Trailers: '09 Wildwood LE 27RB, '02 Palomino Yearling RL, '87 Jayco 1006dlx